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The widespread variation in brain size and complexity that is evident in sharks and holocephalans is related to both phylogeny and ecology. Relative brain size (expressed as encephalization quotients) and the relative development of the five major brain areas (the telencephalon, diencephalon, mesencephalon, cerebellum, and medulla) was assessed for over 40 species from 20 families that represent a range of different lifestyles and occupy a number of habitats. In addition, an index (1–5) quantifying structural complexity of the cerebellum was created based on length, number, and depth of folds. Although the variation in brain size, morphology, and complexity is due in part to phylogeny, as basal groups have smaller brains, less structural hypertrophy, and lower foliation indices, there is also substantial variation within and across clades that does not reflect phylogenetic relationships. Ecological correlations, with the relative development of different brain areas as well as the complexity of the cerebellar corpus, are supported by cluster analysis and are suggestive of a range of ‘cerebrotypes’. These correlations suggest that relative brain development reflects the dimensionality of the environment and/or agile prey capture in addition to phylogeny.

Several patterns of brain allometry previously observed in mammals have been found to hold for sharks and related taxa (chondrichthyans) as well. In each clade, the relative size of brain parts, with the notable exception of the olfactory bulbs, is highly predictable from the total brain size. Compared with total brain mass, each part scales with a characteristic slope, which is highest for the telencephalon and cerebellum. In addition, cerebellar foliation reflects both absolute and relative cerebellar size, in a manner analogous to mammalian cortical gyrification. This conserved pattern of brain scaling suggests that the fundamental brain plan that evolved in early vertebrates permits appropriate scaling in response to a range of factors, including phylogeny and ecology, where neural mass may be added and subtracted without compromising basic function.T he allometric relationship of brain parts to overall brain size has been studied and debated extensively (1-7). At the core of the debate lies the question of whether the brain is best characterized as a collection of independently varying structures/devices evolved for particular behavioral requirements or niches or as a single coordinated processing structure/device in which adaptation for species-specific behavioral capacities occurs without the production of delineable modules (8, 9). Many methodological issues have arisen as well, including what about a brain should be quantified [cells or volumes (10)], what should be compared and how, and how to take into account the statistical dependence of both structural and species relationships (11).Until recently, a single data corpus comprising primates, bats, and insectivorous mammals was the sole source for comparison (2), leaving the question of whether these mammals represented all vertebrates, or even all other mammals, unresolved. The addition of carnivorous mammals (including marine mammals), ungulates, xenarthrans, and the manatee demonstrated that the original conclusions drawn from primates, bats, and insectivores could be extended to this larger data set (8, 12). These studies revealed that mammalian brain structure exhibits a pattern of variation containing two principal components. The first component, accounting for ≈96% of the total variance of related brain parts to total brain size, loads most highly on neocortex and cerebellum. The second component loads most highly on the olfactory bulb and associated limbic structures and accounts for ≈3% of the original variance. Each brain part also has a characteristic slope with respect to absolute brain size, such that every large mammalian brain is composed disproportionately of neocortex and cerebellum. The remaining 1% of the variance must subsume all remaining sources, including niche, sex and individual differences, and measurement error. This 1% contribution is large in one sense: In two species with the same brain size, a single structure might differ by a factor of 2.5. The total range of structure sizes may differ by a factor of 100,000 or more b...

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A systematic study of the ambient noise in the shallow coastal waters of north-eastern New Zealand shows large temporal variability in acoustic power levels between seasons, moon phase and the time of day. Ambient noise levels were highest during the new moon and the lowest during the full moon. Ambient noise levels were also significantly higher during summer and lower during winter. Bandpass filtering (700-2,000 Hz and 2-15 kHz), combined with snap counts and data from other studies show that the majority of the sound intensity increases could be attributed to two organisms: the sea urchin and the snapping shrimp. The increased intensity of biologically produced sound during dusk, new moon and summer could enhance the biological signature of a reef and transmit it further offshore. Ambient noise generated from the coast, especially reefs, has been implicated as playing a role in guiding pelagic post-larval fish and crustaceans to settlement habitats. Determining a causal link between temporal increases in ambient noise and higher rates of settlement of reef fish and crustaceans would provide support for the importance of ambient underwater sound in guiding the settlement of these organisms.

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